1. Field of the Invention
The invention relates to the field of integrated circuits and, more particularly, to a high frequency, voltage controlled oscillator that can be fully integrated.
2. Description of the Prior Art
Oscillators are astable circuits which find many uses in electronic equipment, both stationary and portable. Oscillators frequently consist of one or two transistors, an inductor (L), and a capacitor (C) in an LC tank circuit, followed by a buffering amplifier. Oscillators have followed the trend toward circuit integration, but external devices are still required for many reasons. Reasons are stability of oscillation requiring special components such as crystal oscillators, or the need of an inductor as part of the LC tank circuit, or the use of a voltage variable capacitor (varactor) also known as variable-reactance diode.
An example of the prior art is disclosed in U.S. Pat. No. 5,055,889 (Beall), which provides a lateral varactor that can be implemented in a MESFET integrated circuit. The fabrication methods for MESFET's are not suitable for bipolar circuits. U.S. Pat. No. 5,187,450 (Wagner et al.) describes a high-frequency voltage controlled oscillator (VCO) that can be implemented on a single integrated circuit (IC), without a varactor, but an external LC network is still required for frequency control. A highly stable, high frequency VCO for phase locked loops is disclosed in U.S. Pat. No. 5,191,301 (Mullgrav, Jr.). Here, the design may be integrated on a single chip; however, voltage control of the frequency is achieved by d.c. differential amplifiers coupled to a series of logic blocks formed in a ring.
In the Oct. 3, 1994 issue of Electronic Design an article was published by Michael A. Wyatt and Larry A. Geis, titled "Varactorless HF Modulator". In it a circuit is described which utilizes the base-charging capacitance of a transistor in conjunction with a high-quality, low-loss, ceramic coaxial shorted quarter-wave transmission line. Sustained oscillations are produced when a negative resistance "seen" at the base of the transistor reacts with the transmission line.